Liquid filters, in particular oil filters, typically have a so-called ring filter element by means of which a liquid is filtered which flows through the liquid filter. Said filter element has to be replaced according to a schedule to be able to ensure the filter effect of the liquid filter at all times. During replacement of the ring filter element, the liquid filter is opened, for example, by removing a filter housing cover so as to provide access to the ring filter element. Due to the constantly increasing environmental requirements, the replacement of the filter element must be possible without liquid, in particular oil, escaping into the environment. For this reason, conventional liquid filters have a housing which comprises, besides an inlet channel which communicates with the crude side of the ring filter element, and an outlet channel which communicates with the pure side of the ring filter element, a drainage channel by means of which a housing space, in which the ring filter element is arranged, can be drained when opening the filter housing. Of course, said drainage channel has to be closed when the liquid filter is ready for operation because otherwise the pure side and the crude side of the ring filter element would be interconnected and thus, the liquid filter would not be able to develop its filter effect.